Biaxial stretch blow molding devices have so far been known as devices for producing hollow containers made of synthetic resins, such as plastic containers. In the biaxial stretch blow molding device, it is common practice that a preform is transported to a blow mold by a rotating disk, with a neck of the preform being held; the preform is stretched by a stretch rod within the blow mold; and high pressure air is fed into the preform via a blow core mold for closing a mouth (opening) of the preform, whereby a hollow container is blow-molded. In this manner, a hollow container of a desired shape can be formed.
The biaxial stretch blow molding device is configured, for example, as follows: A core fixing plate to which a blow core mold is fixed is raised or lowered (moved forward or backward), together with an elevation block, by a first air cylinder provided fixed block. An elevation plate to which a stretch rod is fixed is raised or lowered by a second air cylinder provided in the elevation block (see, for example, Patent Document 1).
In forming a hollow container such a biaxial stretch blow molding device, the stretch rod is moved (lowered) by the second air cylinder, with the movement (e.g., descent) of the blow core mold being stopped. Since air is compressed or expanded, it is difficult to exercise fine control of speed simply by the air cylinder. Thus, if it is attempted to operate the first air cylinder and the second air cylinder synchronously in the device described in Patent Document 1, for example, deviation between their timings may exert an influence to induce damage to the device.
Generally, in order that the stretch rod has been inserted to a site near the bottom of the preform at the completion of the movement (descent) of the blow core mold, the stretch rod is lowered beforehand to a predetermined position conformed to the length of the preform, at a standby (ascent) position before descent of the blow core mold. That is, at the standby position, the stretch rod is protruded with a predetermined length below the blow core mold. By so doing, a cycle time taken for blow molding can be shortened.
In this case, however, in order to avoid interference between the stretch rod and the rotating disk for transporting the preform, there is need to set a position upwardly apart from the rotating disk by an amount, which is equal to or more than the amount of protrusion of the stretch rod from the blow core mold, as the standby position of the blow core mold.
To close the neck mold for the preform with the blow core mold, therefore, it is necessary to lower the blow core mold by at least a distance corresponding to the amount of protrusion of the stretch rod. That is, the stroke amount of the blow core mold during ascent or descent becomes relatively large. As a result, the problem occurs that the time taken for ascent or descent of the blow core mold lengthens, resulting in a long cycle time taken by blow molding.
To deal with such a problem, a device is available in which a drive source for the blow core mold or the stretch rod is changed from a pneumatic cylinder to a servo motor to achieve a reduction in the cycle time. A concrete example is a device which is equipped with a first servo motor and a second servo motor, and in which when the second servo motor is stopped, the first servo motor is driven to move a blow core fixing plate and a stretch rod fixing plate forward or backward; or when the first servo motor is stopped, the second servo motor is driven to move the stretch rod fixing plate forward or backward independently (see, for example, Patent Document 2).
By moving the blow core mold and the elevation rod forward or backward (upward or downward), as described above, the time taken for the ascent or descent of the blow core mold and the elevation rod shortens, and can thus reduce the cycle time.